Spatial Light Modulators with Changeable Phase Masks for Use in Holographic Data Storage

ABSTRACT

A holographic data storage system that includes a write head that includes a pixellated spatial light modulator and a separate or integral phase mask that varies the phase depending on the location in the phase mask that light passes through. The phase variation can be changed over time in a random, pseudo-random, or predetermined fashion. The spatial light modulator and phase mask can be implemented in a liquid crystal SLM (nematic, ferroeleletric, or other), in a DMD SLM, in a magneto-optical SLM, or in any other suitable manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/739,642 filed on Apr. 24, 2007, entitled “Spatial Light Modulatorswith Changeable Phase Masks for Use in Holographic Data Storage” whichclaims priority from U.S. Provisional Patent Application Nos. 60/745,505filed Apr. 24, 2006, entitled “Spatial Light Modulators with IntegratedPhase Masks that can be Changed with Time,” and 60/883,492 filed Jan. 4,2007, entitled “Digital Display,” the contents of each of which areincorporated herein by reference.

BACKGROUND

Holographic optical data storage is an attractive alternative tomagnetic tape, magnetic disc, and optical disc storage of digitalcomputer data. It offers high capacity and high recording and readingdata rates on storage media that can be removed from the drive, asdescribed in Holographic Data Storage, H. J. Coufal, D. Psaltis, G. T.Sincerbox, editors, (Springer-Verlag, Berlin, 2000), incorporated hereinby reference. Data to be stored is written to a photosensitive storagemedia by overlapping an information-bearing light beam (the signal beam)with a reference light beam. When the beams are coherent, coming forexample from the same laser, standing waves in the beam's interferencepattern create changes in the photosensitive material's index ofrefraction, thus forming a hologram. The stored data can be read out byilluminating the recorded hologram with the reference beam alone: thehologram diffracts light from the reference beam to create a copy of theoriginal information-bearing beam. Multiple holograms can be recordedwithin the same volume of storage media by, for example, varying theangle of the reference beam. This is known as angular multiplexing. Manyother hologram-multiplexing techniques are known in the art. The use ofvolumetric storage enables extremely high capacities, and theparallelism inherent in page-oriented storage offers much higher datarates that conventional serial bit-at-a-time technologies.

The information to be recorded or stored is imposed on the light beamthrough the use of a spatial light modulator (SLM). The SLM convertsinput electronic data to a two-dimensional image of bright and darkpixels, for example. Light modulated by the SLM passes through theoptical system of the HDS device or drive to be recorded within thestorage medium. In some instances, the SLM may modulate the phase(rather than the intensity or amplitude) of the light. Typically, a lensbetween the SLM and the recording medium is used to form a spatialFourier transform of the SLM image in the region where the hologram isto be recorded in the photosensitive material of the storage medium.Subsequently, when it is desired to read the data stored in the medium,the hologram stored in the recording medium is illuminated by thereference beam to reconstruct the SLM image, which can then be detectedby a photodetector such as a CCD camera. One example of an SLM suitablefor holographic data storage systems can be made using ferroelectricliquid crystals (FLCs) atop a CMOS backplane, constructed similarly tothe microdisplay devices described in U.S. Pat. Nos. 5,748,164 and5,808,800, the contents of which are incorporated herein by reference.These SLMs can be fabricated by techniques that are well known in theart, for example as described in “Semiconductor manufacturing techniquesfor ferroelectric liquid crystal microdisplays,” by Mark Handschy inSolid State Technology volume 43, pages 151-161 (2000), incorporatedherein by reference.

However, several difficulties in the implementation of a practicalholographic data storage system can be traced to the design andperformance of the signal-beam optical path. Also, the particular FLCSLM devices described in the abovementioned patents do not make idealwrite-heads. For example, when the SLM is operated as an intensitymodulator, its Fourier transform contains a bright central spot, the DCspot, that is as much as 60 dB (one million times) brighter than thesurrounding light intensity. This bright spot can saturate the opticalrecording medium, making it difficult to record and reconstruct datawith high fidelity.

It is known in the art that the Fourier-plane DC bright-spot problem canbe solved by introducing into the optical system a phase mask thatimposes fixed, pseudo-random optical phase variations across the wavefront, as is disclosed in U.S. patent application Ser. No. 11/046,197,“Phase Masks for Use in Holographic Data Storage,” incorporated hereinby reference. That patent application disclosed the fabrication of phasemasks by a variety of techniques including relief structures in eitherthe window or mirrors of a liquid crystal on silicon (LCOS) SLM. Alsodisclosed there was the implementation of an integral phase mask by useof three or more electrically selected states of the liquid crystalmodulators in a liquid-crystal-on-silicon (LCOS) SLM.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

SLMs are described herein with integrated phase masks wherein the phasespatial pattern of the phase mask may be changed in time. This hasparticular benefits for the use of such SLMs as the input device in anHDS system, where varying the SLM phase pattern reduces the build up inthe HDS recording media of the otherwise stationary pattern of sharpintensity peaks produced by the SLM. Thus, a more even exposure of therecording media is produced, which reduces the demand on media dynamicrange, and permits recording a greater number of holograms per unitmedia volume, and hence storage of data at a greater density.

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools, and methods which aremeant to be exemplary and illustrative, and not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

A write head for a holographic data storage system includes a spatiallight modulator that is arranged into an array of pixels that can eachseparately modulate light that is directed toward the spatial lightmodulator; and a phase mask that is capable of varying the phase oflight passing therethrough, the phase variation being dependent on theparticular portion of the phase mask that the light passes through. Thephase variation of at least a portion of the phase mask can be changedover time.

The pixels of the spatial light modulator may be switched into one of atleast three different optical states. The spatial light modulator may bea liquid crystal spatial light modulator. The spatial light modulatormay be a ferroelectric liquid crystal spatial light modulator. Thespatial light modulator may include a layer of liquid crystal materialsandwiched between two substantially planar surfaces.

The spatial light modulator may include micromechanical mirrors that canbe moved over time. Each of the micromechanical mirrors may be tilted toat least one of two different positions to turn the pixel associatedwith that mirror ON or OFF and each of the micromechanical mirrors aremoved in a direction substantially parallel to the incoming light to atleast one of two different positions to change the phase of the pixelassociated with that mirror.

Each pixel in the array of pixels of the spatial light modulator mayinclude a pixel electrode and each pixel electrode includes at least twosegments, wherein the two segments can be driven to different voltagelevels relative to each other. The two segments may include interleavedportions that can create a diffraction grating.

The phase variation may be changed in a random or pseudo-random fashion.The phase variation may be changed in a predetermined fashion. Thepixels of the spatial light modulator may have an amplitude opticalstate determined by data supplied thereto, wherein the predeterminedfashion is determined in accordance with the supplied data. Theamplitude optical state may include at least two different amplitudeoptical states, wherein for one of the two different amplitude opticalstates, the phase variation is of a first amount for a given pixel, andfor the next subsequent pixel that is in that same amplitude opticalstate, the phase variation is of a second amount, with the phasevariation continuing to alternate between the first and second amountsfor subsequent pixels.

The array of pixels in the spatial light modulator may have a pixelpitch of less than 6 um. The phase variation may be determined by aphase mask generator that is located in a silicon backplane with thespatial light modulator.

In another aspect, a write head for a holographic data storage systemincludes a spatial light modulator that is arranged into an array ofpixels that can each separately modulate light that is directed towardthe spatial light modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through. The phase variation of at least a portion of thepixels can be changed over time.

In another aspect, a write head for a holographic data storage systemincludes a spatial light modulator having an array of pixels, each ofthe pixels in the array being independently switchable between a highoptical throughput state and a low optical throughput state in responseto data supplied thereto, each of the pixels in the high opticalthroughput state further being switchable between a first optical phasestate and a second optical phase state in a predetermined fashion inaccordance with the supplied data.

Each pixel may include a switchable diffractive structure switchablebetween a first state of greater diffraction and a second state oflesser diffraction. The high optical throughput state may correspond tothe second state of lesser diffraction while the low optical throughputstate corresponds to the first state of greater diffraction.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

DRAWING DESCRIPTIONS

FIG. 1 is a perspective view of a holographic data storage system.

FIG. 2 shows the Fourier plane of an SLM with a fixed binary phase mask,along with intensity profiles taken horizontally and vertically throughthe center spot.

FIG. 3 is a perspective view of an SLM including a phase mask.

FIG. 4 is a schematic side view of portions of the SLM of FIG. 3,showing a 3-state driver for the pixel electrodes.

FIG. 5 is a view of the three optical states possible from the SLM ofFIG. 3.

FIG. 6 is a schematic view of a polarizer, 3-state SLM, and analyzer.

FIG. 7 is a schematic of a pixel utilizing a diffractive structure toachieve ternary modulation.

FIG. 8 is an FLC analog charge-control drive circuit.

FIG. 9 shows an improved electrode arrangement for a diffractive ternaryFLC

SLM.

FIG. 10 is a view of a pixel electrode structure for a diffractiveternary FLC SLM.

FIG. 11 shows a pixel circuit for driving a diffractive ternary FLC SLMpixel.

FIG. 12 shows FLC optical response times at various temperatures whendriven with ±1.65 V electrical drive.

FIG. 13 shows a system for producing a data pattern on an SLM.

FIG. 14 shows another system for producing a data pattern on an SLM.

FIG. 15 shows a drive circuit for an LCOS device in a ternary SLM.

DETAILED DESCRIPTION

Reference will now be made to the accompanying drawings, which assist inillustrating the various pertinent features of the present invention.Although the present invention will now be described primarily inconjunction with holographic data storage applications, it should beexpressly understood that the present invention may be applicable toother applications where altering an image with a phase mask isrequired/desired. In this regard, the following description of animproved holographic data storage system is presented for purposes ofillustration and description. Furthermore, the description is notintended to limit the invention to the form disclosed herein.Consequently, variations and modifications commensurate with thefollowing teachings, and skill and knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedherein are further intended to explain modes known of practicing theinvention and to enable others skilled in the art to utilize theinvention in such, or other embodiments and with various modificationsrequired by the particular application(s) or use(s) of the presentinvention.

FIG. 1 shows the optical arrangement of a holographic data storagesystem 20 according to the present invention. As can be seen, a beam 22from a coherent light source, such as a laser, is split by a beamsplitter 24 into two beams, a signal beam 26 and a reference beam 28.The signal beam 26 is directed toward a spatial light modulator 30, suchas a ferroelectric liquid crystal modulator as exemplified by thoseproduced by Displaytech, Inc. of Longmont, Colo. Alternatively, thespatial light modulator (SLM) could be any other suitable type of SLMwhether ferroelectric liquid crystal or other type of liquid crystals,such as nematic liquid crystal, or a MEMS (MicroElectroMechanicalSystem) device such as a digital micromirror device (DMD) such as thoseproduced by Texas Instruments under the trademark DLP or those producedby Silicon Light Machines (now a subsidiary of Cypress Semiconductor)under the name Grating Light Valve, or other MEMS device, or any othersuitable SLM such as a semiconductor multiple quantum well (MQW) SLM ora magneto-optic SLM such as those formerly produced by Litton.

One of the reference beam 28 and the signal beam 26 (in this case thereference beam) is directed off-axis where it is then re-directed towardan optical storage medium 32, that may be composed of any suitablephotosensitive material (such as a photorefractive crystal or aphotopolymer like those available from InPhase Technologies, Inc. inLongmont, Colo. or from Aprilis, Inc. in Cambridge, Mass.). The lightfrom the signal beam 26 is modulated by the SLM 30 (operating intransmission or reflection) and directed to the same point in thestorage medium 32 as well. After modulation by the SLM, the signal beam26 inteferes with the reference beam 28 within the optical storagemedium 32 to form a three-dimensional hologram, representative of allthe data displayed by the SLM 30. This is the process that is followedto write data to the storage medium 32 with the holographic data storagesystem 20. The stored hologram may be representative of an image or of apage of data. As can be appreciated with an SLM having an array of 1000by 1000 pixels, 1,000,000 bits of data can be stored in each hologram.

In order to read data from the storage medium 32 with the holographicdata storage system 20, the signal beam 26 and spatial light modulator30 are not needed and the reference beam 28 alone illuminates thehologram. A detector 34, which may be in the form of a detector array,may be located on the opposite side of the storage medium 32 to read theimage that is produced when the hologram is illuminated by the referencebeam 28. In this manner, the image can be read back to either displaythe image or to read the data bits stored in that page of data. As canbe appreciated, data can be read back much faster than inone-bit-at-a-time optical storage systems like CD-ROM drives.

In one form of holographic data storage system, the detector is an arrayof intensity-detecting pixels such as a CCD camera sensor. In this case,the image reconstructed by illuminating the hologram with the referencebeam alone should be an image having intensity varying in accordancewith the recorded data values. Such an intensity image is produced ifthe SLM used to record the hologram produced intensity modulation of thesignal beam in accordance with the input data to be recorded. Forexample, a hologram readable by an intensity detector would be producedby an SLM that produced a bright or ON pixel when a data value of 1 wasinput to that pixel and produced a dim or OFF pixel in response to a 0being input. This is in contrast to recording a pure phase or phase-onlyhologram. Such holograms do not suffer from the DC hot spot problem, butdetecting the reconstructed image is more complicated.

A single exposure with laser light in the recording media of an HDSsystem, which media typically encompasses or is near to the Fourier planof the input SLM, generally produces an intensity pattern having sharpintensity peaks similar to the well-known laser phenomenon of speckle.Such a pattern, produced by exposing an LCOS SLM, fabricated at taughtin the '197 patent application with a phase-mask integrated into itswindow, and recorded in its Fourier plane, is shown in FIG. 2, where the“fuzzy” white pattern shows the speckle-like intensity pattern. Theintensity profiles shown adjacent to the bottom and left edge of thefigure are taken along the cursor lines which pass through the DC hotspot, which arises because the desired π-value for the phase depth of abinary phase mask was not achieved in this particular SLM. In additionto the DC hot spot, there are large peaks (three of which are pointedout) in the intensity pattern of the light scattered away from the DCposition by the phase mask.

Although it is typical in HDS data recording to multiplex multipleholograms into a single volume of media by slight shifts, for example,in the angle of the reference beam (“angle multiplexing”) or in theposition of the SLM relative to the media (“shift multiplexing”), thecharacteristic pattern of intensity peaks produced by the particularphase-mask pattern of an SLM as disclosed in the '197 patent applicationmay not be changed so much from one multiplex exposure to the next toprevent a given peak on one exposure from overlapping with that samepeak on a subsequent exposure.

This phenomenon results in the recording media in some spots being moreheavily exposed than in other spots, which in turn limits the number ofexposures that can be recorded in a given volume of media. By changingthe phase pattern on the SLM from exposure to exposure or from time totime, the pattern of intensity peaks in the recording plane can also bevaried. Then the exposure of the media tends to average to a moreuniform value, and the limits on exposure number and data densityotherwise encountered can be avoided.

Thus, it may be desirable to have an SLM that can produce intensity oramplitude modulation, to enable detection by a common intensity sensorarray. It may further be desirable that the SLM provide a varyingoptical phase in the form of a phase mask, in or close to the plane ofthe pixels, to spread out or diffuse the optical energy that wouldotherwise make a DC hot spot. Finally, it may be desirable that the SLMenable the phase mask pattern to be selectably changed from exposure toexposure and in response to the input data pattern, to avoid coherentbuild up of bright spots in the recording media that would unnecessarilyreduce media data capacity, while still providing the high-contrastintensity modulation needed for detection of the replayed input data.SLMs that could provide the needed multi-state phase-plus-amplitudemodulation are described in the '197 patent application and in priorart.

FIGS. 3-5 show an SLM 150 as disclosed in the '197 patent application.As shown in FIG. 3 and FIG. 4, the SLM 150 includes a silicon chip 152with an array of pixel mirrors/electrodes 154 defined thereon. A layerof FLC material 156 is sandwiched between the silicon chip 152 and aglass window 158, which may have an ITO transparent conductor layer 160and an alignment layer (not shown). Another alignment layer (not shown)may be placed on top of the pixel mirrors/electrodes 154. A 3-statedriver 162 for one of the pixel mirrors/electrodes 154 is shown in thesilicon chip 152 in FIG. 4.

The three light modulation states are demonstrated in FIG. 5 in whichthe optic axis of the FLC is shown in each of the three different driveconditions (optic axes 164, 166, and 168). The at least three statesinclude one state 164 of very low output light amplitude or intensity(the OFF state) and two other (ON) states 166 and 168 of high outputintensity, at least two of which have different optical phase. This isreferred to as ternary modulation, regardless of whether the totalnumber of pixel optical states is three or more than three.

This is illustrated more specifically in FIG. 6, in which the switchableFLC film 170 of an SLM pixel is oriented between crossed polarizers (apolarizer 172 and an analyzer 174) so that the FLC optic axis can beswitched to either side of the incident light polarization direction. Inthe example shown here, the two FLC optic-axis orientations are equallydisposed on either side of the incident polarization, the output lighthas the same intensity for each state, but opposite phase (that is, thephase of one output state is +π/2, and that of the other state is −π/2).The intensity of these two output states is maximized, as usual, whenthe SLM's FLC film 170 has half-wave total retardance—i.e. quarter-wavesingle-pass retardance for a reflective SLM. The output intensity isfurther maximized if the two FLC optic-axis orientations are fully 45°on either side of the input polarization. A third state of near-zerooutput intensity (an OFF state) can be obtained from an FLC optic-axisorientation substantially parallel to the incident polarization. Analogvariation of FLC optic-axis orientation with applied electrical drivesignal is known, for example, in the electroclinic effect, thedeformable-helix effect, and in the so-called “V-shaped” switchingeffect [see, for example, Michael J. O'Callaghan, “Switching dynamicsand surface forces in thresholdless “V-shaped” switching ferroelectricliquid crystals,” Physical Review E, volume 67, paper 011710 (2003);Michael J. O'Callaghan, Michael Wand, Chris Walker, William Thurmes, andKundalika More, “High-tilt, high PS, de Vries FLCs for analogelectro-optic phase modulation,” Ferroelectrics, volume 343, pp. 201-207(2006); the contents of both of which are incorporated herein byreference]. The three optic-axis states shown in FIG. 5 can then beobtained by applying three different levels of electrical drive to apixel of an SLM having suitable analog response.

An SLM having at least the three optical states described with referenceto FIG. 6 can solve the Fourier-plane DC bright-spot problem in a mannersimilar to that of more usual planarized relief phase masks described inthe '197 patent application. Pixels where it is desired that the outputintensity be zero are written with the electrical level that producesthe OFF state having the FLC optic axis parallel to the incidentpolarization. Pixels that are desired to be ON can be written to eitherof the other two states described with reference to FIG. 11. The choicebetween the two ON states, the +π state or the −π state (hereinafter the+ON state and the −ON states, respectively), can be made in exactly thesame way as the prescription for the design of a fixed phase mask.

Another way to implement a three-state switching of the SLM describedabove is also disclosed in the '197 patent application as shown in FIG.7. As has been previously disclosed in U.S. Pat. Nos. 5,182,665 and5,552,916, the contents of which are incorporated herein by reference,switchable diffraction can be produced in conventional binary FLCdevices, and used to modulate light. In fact, it can be used, asdisclosed in the '197 patent application, to produce combined phase andamplitude modulation in a way that allows such an SLM to function toreduce the Fourier-plane DC bright-spot problem in a way exactlyanalogous to that of those with relief-type phase masks. Each pixelelectrode 180 is divided into two interleaved segments, a segment 182and a segment 184, as shown in FIG. 7, each segment having width q/2.For two signs of applied electrical drive, each pixel then has fourstates, as enumerated below.

SEGMENT A SEGMENT B PIXEL STATE + + +ON − − −ON + − OFF − + OFF

The first two columns of the table above show the polarity of theelectrical drive applied by the segment electrodes to the overlying FLC.When positive drive voltage is applied to both segments, their FLCorientations are parallel, both lying on the same side of the incidentpolarization direction, producing the same +ON state as described abovewith respect to analog FLC modulators. When negative drive voltage isapplied to both electrode segments, their FLC orientations are againparallel, but now both on the other side of the incident polarizationdirection, producing the same −ON state as described above for theanalog FLC modulators. When voltages of opposite polarities are appliedto the two segments, the associated FLC material is switched to haveopposite optic axis directions, and a “grating” is produced. Thisgrating will diffract at least part of the incident light, at anglesthat are larger than or equal to β, where sin β˜λ/q according to theabovementioned U.S. Pat. Nos. 5,182,665 and 5,552,916. In a typicalholographic data storage system recorded data density is maximized byinserting a spatial filter into the optical system that removes lightdiffracted from the SLM pixel array into orders higher than the zeroorder. In the case of the sub-pixel gratings shown in FIG. 7, such aspatial filter would essentially stop the light diffracted by the pixelgrating state, thereby producing a dark OFF-state image for pixels sodriven.

Thus, the '197 patent application disclosed two embodiments where thephase pattern was produced by the electrical drive to the SLM pixel. Ina first embodiment case, described in the '197 application withreference to the present FIGS. 3 and 4, a liquid crystal optic axiscould be driven to three orientations, one of which caused the pixel tobe OFF, while each of the other two optic axis states caused the pixelto be ON, but with a phase difference of π from one ON orientation tothe other. In a second embodiment, described with reference to thepresent FIG. 7, a pixel has an electrically switchable diffractiongrating, with two OFF states that diffract light out of the aperture ofthe HDS optical system, and two ON states with a π phase differencebetween them.

Alternatively, it is known to make SLMs that can suitably modulateintensity with selectable phase from nematic liquid crystal devices,typically combined with fixed wave plates, as taught for example byJu-Seog Jang and Dong-Hak Shin in “Optical representation of binary databased on both intensity and phase modulation with a twisted-nematicliquid-crystal display for holographic digital data storage,” publishedin Optic Letters, Vol. 26, No. 22, pages 1797-1799 Nov. 15, 2001, andalso as taught by Judit Reményi, Péter Várhegyi, László Domján, PálKoppa, and Emõke Lõrincz, in “Amplitude, phase, and hybrid ternarymodulation modes of a twisted-nematic liquid-crystal display at ˜400nm,” published in Applied Optics, Vol. 42, No. 17, pages 3428-3434, 10Jun. 2003. Further relevant teaching may be found in the Ph.D. thesis ofLászló Domján, “Generation of Spatial Light Distributions,” Departmentof Atomic Physics, Faculty of Natural Sciences, Technical University ofBudapest, 2004. Each of these publications is incorporated herein byreference.

Further, it is known that magneto-optic SLMs can deliver binary phasestates and a zero amplitude state as taught by Brian A. Kast, Michael K.Giles, Scott D. Lindell, and David L. Flannery, in “Implementation ofternary phase amplitude filters using a magnetooptic spatial lightmodulator,” published in Applied Optics, Vol. 28, No. 6, pages1044-1046, 15 Mar. 1989, which is also incorporated herein by reference.In this case, a magneto-optic film exhibits domains that, through theFaraday effect, rotate the polarization of incident light. Two types ofdomains exist, one that rotates polarization clockwise, and another thatrotates light counterclockwise. Normally, SLM pixels are switched untila single domain has grown to cover the entire extent of the pixel.However, to achieve ternary modulation, Kast et al. partially switched apixel desired to be OFF, producing a mixed state with the two types ofdomains randomly distributed throughout the pixel area. With an analyzercrossed to the polarization of an incident beam, the two types ofcompletely switched pixels will both be ON, but have opposite phase. Amixed pixel can be made to be OFF by following the SLM with a spatialfilter which rejects the light from the high spatial frequencymixed-domain pattern.

It is also feasible that the same effects could be produced by amicro-mechanical (MEMS) SLM. For example, it is well known in the art toproduce an SLM with intensity modulation in the form of an array oftiltable micro-mirrors as is used by the Texas Instruments DLP products.It is known in the art that an SLM with phase modulation can be producedby an array of mirrors that move in piston fashion. An SLM capable ofboth intensity and phase modulation as desired to produce the temporallyvariable phase masks according to the present invention could befabricated by first producing an array of MEMS piston-motion structures,and the fabricating on top of each piston a tiltable mirror. Thus, theON/OFF intensity pattern desired to record the user data would beproduced by electrically driving the tiltable mirrors to selectablydeflect light out of the system aperture, while a superimposed phasepattern could be produced by the driving the piston structures up ordown, desirably to produce a total path length variation of 0 to π. SuchMEMS structures with independent control of mirror height and tilt aredisclosed by Kebin Li, Uma Krishnamoorthy, Jonathan P. Heritage, andOlav Solgaard in “Coherent micromirror arrays,” published in OpticsLetters, Vol. 27, No. 5, pages 366-368, Mar. 1, 2002, which is alsoincorporated herein by reference.

It is also known to use MEMS devices to produce SLMs with selectablephase and amplitude modulation by forming “macropixels” from a pluralityof separately switchable MEMS sub-elements, as disclosed in U.S. Pat.No. 5,312,513 (the '513 patent), which is also incorporated herein byreference. For example, with reference to FIG. 3 therein is taught amacropixel composed of four phase-modulating elements. Each of the foursub-elements has the same area, and each can be switched between twophase states A and B that differ by π; two sub-elements can beselectably switched between a phase state of 0 or π—the other twosub-elements can be switched between a phase state of π/2 or 3π/2; the Aand B states available to each sub-element are listed in the tablebelow.

SUB-ELEMENT STATE A STATE B 1 0 π 2 0 π 3 π/2 3π/2 4 π/2 3π/2It is further taught therein that if the optical system resolution isset to mix the response of the sub-elements into a single response,different phase and amplitude modulations levels can be achieved. Thesystem resolution limitation needed to produce response mixing could beproduced, for example, by introducing a spatial filter that would passspatial frequencies high enough to allow the macropixel to be resolvedbut would cut off the higher spatial frequencies needed to resolve thesub-elements. In the case described with reference to FIG. 3 in U.S.Pat. No. 5,312,513, a zero amplitude state is produced when thesub-elements of the first pair (sub-elements 1 and 2 in the table above)are switched opposite of each other and when the sub-elements of thesecond pair (sub-elements 3 and 4) are also switched opposite of eachother; that is when sub-element 1 is in its A state, sub-element 2 is inits B state, sub-element 3 is in its A state and sub-element 4 is in itsB state. This and three other zero-amplitude states are listed in thetable below:

ZERO-AMPLITUDE STATES SUB-ELEMENT 1 2 3 4 1 0 π 0 π 2 π 0 π 0 3  π/2 π/2 3π/2 3π/2 4 3π/2 3π/2  π/2  π/2As can be seen from this table or from FIG. 4 in the '513 patent, zeroamplitude is achieved by setting equal-area pairs of MEMSphase-modulating sub-elements to states where members of a pair differin phase by π.

The macropixel described with reference to FIGS. 3 and 4 in the '513patent can also produce a variety of states with non-zero amplitude,including non-zero amplitude states that differ in combined phase by π.Four such states all in this case having amplitude equal to 0.707 arelisted in the table below.

NON-ZERO AMPLITUDE STATES SUB-ELEMENT 5 6 7 8 1 0 π π 0 2 0 π π 0 3 3π/2π/2 3π/2 π/2 4 3π/2 π/2 3π/2 π/2 COMBINED PHASE 7π/4 3π/4  5π/4 π/4COMBINED AMPLITUDE 0.707 0.707 0.707 0.707States 5 and 6 differ in phase by π, as do states 7 and 8. Thus, thedesired ternary modulation could be achieved by choosing any of states1, 2, 3, or 4 for the OFF state, choosing state 5 for the +ON state andstate 6 for the −ON state. Alternately, states 7 and 8 could be chosenfor the +ON and −ON states. The same principles can be applied to otherMEMs devices where macropixels are formed from sub-elements, such as theGrating Light Valve disclosed in U.S. Pat. No. 7,057,795: in an opticalsystem of resolution limited so that a pixel is resolved but itssub-elements are not an OFF state is produced by pairs of sub-elementsof equal area switched so the members of the pair differ in phase by π,while + and −ON states are produced by setting the sub-elements so themembers of the pair do not differ in phase.

The two ternary FLC SLMs described in the '179 patent application, ananalog-responding FLC SLM and a diffractive FLC SLM, can be furtherimproved for use in the present invention. In the case of the analogSLM, the SLM analog drive circuitry deserves further consideration. Itis taught in the two O'Callaghan references mentioned above thatsuperior control over the FLC analog response can be achieved usingcharge control drive. A charge-control drive circuit of the type used inthese references is shown here in FIG. 8. The FLC element, portrayedhere as a capacitor, provides the negative feedback around anoperational amplifier. An input voltage V_(in) causes a chargeQ=V_(in)C_(ref) to appear across the FLC element. To make a ternary SLMemploying analog FLC modulators, an LCOS device could be made with eachpixel containing a drive circuit similar to that shown in FIG. 8,suitably modified to allow one terminal of each FLC pixel to connect toa common electrode (ITO glass window) not accessible to the individualpixel driver, as shown in FIG. 15, which measures the voltage across areference capacitor 801 using a difference amplifier 803, and thenapplies negative feedback using amplifier 805 to make thereference-capacitor voltage equal to an input voltage 807, thus ensuringagain that the FLC element 809 is driven with a charge Q=V_(in)C_(ref).

Simpler pixel drive circuits could be made using current sources derivedfrom current mirrors, as is know in the art. To drive the FLC pixel witha desired amount of charge it is simply required to connect a currentsource of known strength to the FLC pixel for an amount of time neededfor the source to supply the desired charge. For the ternary FLC pixelsdesired here as part of an SLM with changeable phase mask, the two ONstates could be achieved by conventional voltage drive (with the pixelelectrode being driven to one or the other power supply rail dependingon an input bit indicating whether the pixel state should be + or −ON),with the current source being used only to provide the charge needed todrive the FLC to the OFF state.

Alternately, charge-control drive for analog ternary FLC pixels can beprovided without recourse to any analog drive circuitry. The reversal ofthe FLC's ferroelectric polarization provides a predictable current sinkin response to a drive-voltage switching edge. To drive an FLC pixel tothe OFF state at the center of its range between the two extreme ONstates it is only necessary to open-circuit the pixel electrode at theswitching half-way point. This can be accomplished in a conventionalpixel driver by inserting a CMOS transmission gate between the voltagedrive and the pixel electrode. Then, in the case where it is desired toswitch the pixel from one ON state to the other, the transmission gatecan be left closed, whereas when it is desired to switch from one ONstate to the OFF state, the transmission gate can be opened at thehalf-way point according to a globally distributed timing signal.Whether the gate opens or not can be controlled by a single bit storedin the pixel (the bit indicating OFF). This disadvantage of thistechnique is that it relies on the uniformity of the response dynamicsof all the pixels across the SLM. Small variations in SLM manufacturingmay produce non-uniformities. For example, the liquid-crystal gap may bethicker at the center of the SLM and thinner at its periphery. In thiscase, pixels in the center may switch slower than pixels at the edge. Inthe referenced U.S. provisional patent application 60/883,492 isdescribed means for controlling the timing of pixel switching eventswith a very high degree of precision while maintaining a very compactpixel layout. This technique could easily be adapted for the presentpurposes by using the TRIGGER signal to control the state of a pixeltransmission gate rather than the state of the voltage drive. Then, inaddition to data representing the input data, compensating values couldalso be stored in the pixel, to provide correct timing for achieving ahigh-contrast OFF state. The compensating data could derived bycharacterizing the response of an individual SLM, on a pixel-by-pixelbasis, if necessary.

The diffractive ternary SLM described in the '170 patent application asshown here in FIG. 7 needs only a single metal layer to provideelectrical connection to the two interdigitated electrodes. Alternately,the structure in FIG. 9 could produce a higher-contrast OFF state. Inthe structure shown in FIG. 9, the pixel electrode is divided into sixstripes or sub-elements. The stripes are connected to two underlyingdrive terminals by vias, indicated here by black dots. Three of thestripes are connected to a first drive terminal, and the other threestripes are connected to a separate drive terminal The drive terminalscan be driven as described above with reference to FIG. 7; however, byextending the stripe structures almost all the way to the edge of thepixel (the edge indicated here by the dashed line) the diffractionproperties are improved.

It is also known to make diffractive pixels with sub-elements havingshapes other than stripes, as disclosed in U.S. Pat. No. 7,064,883.Although the modulators disclosed in the patent are MEMS-based, the sameprinciples can be applied to FLC modulators, as described now withreference to FIG. 10, which shows the electrode for a single pixel outof an array of similar or identical pixels. The pixel electrode isdivided into two separately switchable sub-elements, one a perforatedsquare (with vertical shading lines in FIG. 10) and the other ninecircles electrically connected together (with horizontal shading linesin FIG. 10). These two sub-elements can again be driven as describedabove with reference to FIG. 7 to produce + and −ON states differing byit in optical phase, and two OFF states resulting from spatial low-passfiltering in the optical system rejecting the light diffracted by thedifferently-switched sub-elements. Best performance is achieved when theelectrode area of the two sub-elements is equal, that is when thevertically-shaded region in FIG. 10 has an area equal to thehorizontally-shaded region. Shapes other than squares and circles can beused, as disclosed in the '883 patent. The pixel shown in FIG. 10 uses a3×3 array of one shape within a square pixel. Alternately, a 4×4 arrayor a 5×5 array or even a 2×2 array could be used. As the number ofelements in the array is increased, the diffracted light produced in theOFF state is diffracted to higher and higher angles which is generallydesired for producing higher contrast between the ON and OFF states.However, given the necessity for an insulating gap separating the twoelectrode regions (shown as the white, un-shaded regions surrounding thecircles in FIG. 10) of a least some minimum dimension, the reflectivefill factor of the overall pixel electrode decreases as the number ofshapes in the array is increased. Thus, contrast and throughput can betraded off to optimize performance given the requirements of aparticular system by changing the number of shapes used to form thepixel sub-elements. It is even possible to use a sub-element composed ofa single shape to make a ternary pixel if only modest contrast wererequired.

Similarly, for the stripe-element ternary pixel described with referenceto FIG. 9, the number of stripe pairs can be varied from three as shown,to four, five, two, or even one. The fill factor and throughputefficiency are improved by narrowing the insulating gap betweenelectrode regions as far as possible. For example, LCOS CMOS processescan produce gaps as small as 0.5 μm or even 0.3 μm. In the case of aternary pixel with n stripe pairs, pixel pitch p and inter-electrode gapg, the fill factor is given by {1−(g/p)[(4n²+1)/(2n)−g/p)]}. For an SLMwith pixel pitch p=10 μm, g/p=0.05 for 0.5 μm gaps. For three pairs ofstripes, the fill factor is 69%, which would improve to 82% with 0.3 μmgaps. Even at a pixel pitch of p=6 μm, a fill factor of 79% can beachieved with 0.3 μm gaps and two stripe pairs.

The diffractive ternary FLC pixels, whether with stripe sub-elements orother shapes, can be driven with a CMOS pixel circuit like that shown inFIG. 11. It comprises four standard cross-coupled inverter staticlatches and four transmission gates. Each sub-element electrode can beconnected to one of two latches according to the state of thetransmission gates. When line A is high (and line nA is low), the eachsub-element is connected to the top one of the respective latches, whilewhen line A is low (and line nA is high), the sub-element is connectedto the bottom one of the respective latches. While the FLC modulatorsare displaying the data associated with the top latch in each pair, thestate of the bottom latch can be changed by asserting new data on theDATA and nDATA lines while activating the appropriate row electrodes.Then, the state of the A lines can be changed to display the newlywritten data while the data in top latch in each pair can be changed ina similar fashion. Given six transistors in each static latch and fourin each transmission gate, this pixel circuit comprises 40 transistors.Applicant has observed that silicon foundries offer static-latchstandard cells that occupy an area of approximately 130 f², where f isthe process feature size or node. Allowing an area for the transmissiongate similar to that for the latch, this pixel circuit could be laid outin an area of about 1040 f ². For a CMOS process with f=0.18 μm, thiswould require an area of 33.7 μm², or a square about 6 μm on a side.Those skilled in the electronic and VLSI arts will appreciate thatsimpler circuits with the same functionality as that shown in FIG. 11are possible, enabling still smaller layouts. It is a virtue of FLCmodulators that they provide fast optical response time with relativelylow drive voltages. For example, applicant has measured the FLC opticalresponse curves shown in FIG. 12. Optical response that essentiallysaturates in 200 μs can be obtained in this example with drive of ±1.65V, as could be provided by CMOS drivers putting out 0 V or 3.3 V, withthe common electrode held at V_(WIN)=1.65 V. Faster FLC materials withresponse times of 100 μs or even 50 μs can be engineered by suitablechoice of molecular constituents and operating temperature, even withdrive voltages constrained to CMOS logic levels such as 5 V or 3.3 V.The circuit of FIG. 11 enables providing new data to the pixel duringthe FLC optical response time, so a new optically-valid state can beobtained every 200 μs, or even every 100 μs, or every 50 μs. Theseresponse speeds are considerably faster than can easily be obtained withnematic liquid crystals like those disclosed by Reményi et al. or byJang and Shin. Further the operating voltages are lower than thoseusually found for MEMS pixels, such as the 10-14 V usually required tooperate devices like the Grating Light Valve, or even the ±26 Vconventionally used to operate the Texas Instruments DLP tiltingmirrors. Lower drive voltages favor reduced operating power and smallerpixel layouts and pixel pitches. FLC LCOS SLM power is overall muchlower than that associated with the high currents needed to drive knownmagneto-optic SLMs. Furthermore, FLC SLMs can be designed to operateover a wide range wavelengths including blue light with wavelength asshort at 400 nm, which is difficult for magneto-optic SLMs because ofthe short-wavelength absorption of the garnet materials on which theyare based.

FIGS. 13 and 14 show two embodiments of systems for producing datapatterns on SLMs, which patterns have superimposed phase-mask patternsthat are changeable. In the system shown in FIG. 13, the SLM acceptsdata from an external source 910, such as the channel electronics in adata storage drive. The SLM comprises an array of ternary modulators asdescribed above. The SLM is preferably implemented as a siliconbackplane, with reflective liquid-crystal or MEMS modulators, or morepreferably FLC modulators. The state of the ternary pixels is controlledby row drive electronics 906 and column drive electronics 904. Theoverall SLM is controlled by a controller 908. The controller 908accepts data from the external source 910; the controller includes aphase-mask generator 912. In response to the sequence of 0's and 1'sprovided from the external source, and in response to internallyexecuted algorithms, the phase-mask controller 908 determines for thepixels that are destined to be turned ON by an input 1, which ones willbe set to a +ON state and which ones will be set to a −ON state.

For example, the phase values for the pixels of an SLM with integratedphase mask produced using ternary modulators could be assigned atrandom. Alternatively, the phase values could be generated by, forexample, a pseudo-random number generator in phase-mask generator 912,electronic implementations of which are well known in the art. The tablebelow shows an example of 16 input data bits: a sequence of 0's and 1'sof data that a user wishes to store in the HDS system.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 input data 0 1 0 0 1 0 1 1 1 0 01 1 1 1 1 input phase x 0 x x π x 0 π π x x π 0 0 0 π

When the user data bit is a 1, which will result in a bright SLM pixel,it is accompanied by a phase value of 0 or π, produced by apseudo-random number generator which is part of phase-mask generator 912(i.e. the phase value need not be provided by the user). When the userdata bit is a 0 which will result in a dark OFF SLM pixel, the phasevalue provided by the pseudo-random generator can be ignored, as denotedby “x”. Using a different phase-mask control algorithm, the phase valuescould be assigned in alternation, as shown in the table below.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 input data 0 1 0 0 1 0 1 1 1 0 01 1 1 1 1 input phase x 0 x x π x 0 π 0 x x π 0 π 0 π

As an alternative to the pseudo-random generation of phase patterns, anumber of fixed patterns could be provided to the SLM, with the fixedpattern being changed from time to time, or even changed every exposure.In this case the SLM cycles through the sequence of patterns. To providegreater diversity of intensity spike patterns in the recording mediawith a smaller set of fixed phase patterns, the phase patterns could beshifted on the SLM. For example, a given phase pattern could be shifteda chosen number of columns to the left across the SLM on every exposureor at some other desired interval. Alternatively, the phase patterncould be shifted down one row of pixels and back the next, or really inany other desired manner.

To reduce the bandwidth required of the SLM data input bus, a preferredembodiment incorporates the pseudo-random number generator into theintegrated circuitry of the phase mask generator 912 which is includedon the SLM backplane. Alternatively, in the case where predeterminedphase patterns are desired, a pattern generation engine could beprovided within the phase-mask generator 912 on the SLM backplane. Thepattern-generation engine could generate phase patterns according toinput “seeds” provided by other elements of the HDS system.Alternatively, predetermined phase patterns could be input to thephase-mask generator and stored in digital memory incorporated into thephase-mask generator 912. These internal memory registers could be readout onto the pixels to determine the phase value a given pixel woulddisplay at a given time. In the case where it is desired to shift thepattern from time to time, circuitry for reading out a single patternfrom the SLM internal memory and writing with a programmable shift valueonto the SLM pixels could also be provided.

In another embodiment, the phase values are not chosen at random. Theintensity spikes or peaks illustrated in FIG. 2 arise from patterns ofphase values in the phase mask that are described by a small range ofspatial frequencies. If the spatial frequencies of the phase mask weremore evenly distributed the height of the spikes or peaks would bereduced. It is possible to design phase masks using suitablecomputational algorithms so that the spatial frequency spectrum is infact more even. However, in an SLM with a fixed phase mask, many of thephase elements will be obscured by OFF pixels at locations that can notbe predetermined since they arise from the arbitrary user data pattern.However, a modified algorithm could compute the phase values in responseto a given data pattern prior to its exposure. The algorithm couldadjust the phase values assigned to pixels having user data value of 1(ON pixels) in a way responsive to the spatial locations of those ONpixels so as to evenly distribute the spatial frequency components ofthe phase pattern and thereby reduce the heights of the intensity spikesin the HDS recording media. This algorithm could be executed byphase-mask generator 912 in response to the input data pattern.

As an alternative to including the phase-mask generator within the SLMbackplane as depicted in FIG. 13, an external phase mask generator 914could be made part of the source 910, as depicted in FIG. 4, thatprovides the data to be stored in the holographic medium. This wouldfacilitate cooperation of the phase-mask controller with the channelelectronics, including any data coding schemes (perhaps iterative) beingexecuted by the channel electronics. The price to be paid is that nowthe amount of input information provided to the SLM will average morethan 1 bit per SLM pixel, creating a need to somewhat increase the inputbandwidth of the SLM.

-   -   Similarly, any light modulating pixel capable of both intensity        or amplitude modulation combined with phase modulation could be        used in accordance with the techniques described herein.

The foregoing description has been presented for purposes ofillustration and description. Furthermore, the description is notintended to limit the invention to the form disclosed herein. While anumber of exemplary aspects and embodiments have been discussed above,those of skill in the art will recognize certain variations,modifications, permutations, additions, and sub-combinations thereof. Itis therefore intended that the following appended claims and claimshereafter introduced are interpreted to include all such variations,modifications, permutations, additions, and sub-combinations as arewithin their true spirit and scope.

1-17. (canceled)
 18. A write head as defined in claim 27, wherein thepixels of the spatial light modulator can be switched into one of atleast three different optical states.
 19. A write head as defined inclaim 27, wherein the spatial light modulator is a liquid crystalspatial light modulator.
 20. A write head as defined in claim 19,wherein the spatial light modulator is a ferroelectric liquid crystalspatial light modulator.
 21. A write head as defined in claim 19,wherein the spatial light modulator includes a layer of liquid crystalmaterial sandwiched between two substantially planar surfaces.
 22. Awrite head for a holographic data storage system, the write headcomprising: a spatial light modulator that is arranged into an array ofpixels that can each separately modulate light that is directed towardthe spatial light modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through; wherein the phase variation of at least a portionof the pixels can be changed over time, wherein the spatial lightmodulator includes micromechanical mirrors that can be moved over time.23. A write head as defined in claim 22, wherein each of themicromechanical mirrors is tilted to at least one of two differentpositions to turn the pixel associated with that mirror ON or OFF andeach of the micromechanical mirrors is moved in a directionsubstantially parallel to the incoming light to at least one of twodifferent positions to change the phase of the pixel associated withthat mirror.
 24. A write head for a holographic data storage system, thewrite head comprising: a spatial light modulator that is arranged intoan array of pixels that can each separately modulate light that isdirected toward the spatial light modulator and that can vary the phaseof light passing therethrough, the phase variation being dependent onthe pixel that the light passes through; wherein the phase variation ofat least a portion of the pixels can be changed over time, wherein eachpixel in the array of pixels of the spatial light modulator includes apixel electrode and each pixel electrode includes at least two segments,wherein the two segments can be driven to different voltage levelsrelative to each other.
 25. A write head as defined in claim 17, whereinthe two segments include interleaved portions that can create adiffraction grating.
 26. A write head for a holographic data storagesystem, the write head comprising: a spatial light modulator that isarranged into an array of pixels that can each separately modulate lightthat is directed toward the spatial light modulator and that can varythe phase of light passing therethrough, the phase variation beingdependent on the pixel that the light passes through; wherein the phasevariation of at least a portion of the pixels can be changed over time,wherein the phase variation is changed in a random fashion.
 27. A writehead for a holographic data storage system, the write head comprising: aspatial light modulator that is arranged into an array of pixels thatcan each separately modulate light that is directed toward the spatiallight modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through; wherein the phase variation of at least a portionof the pixels can be changed over time, wherein the phase variation ischanged in a pseudo-random fashion. 28-29. (canceled)
 30. A write headfor a holographic data storage system, the write head comprising: aspatial light modulator that is arranged into an array of pixels thatcan each separately modulate light that is directed toward the spatiallight modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through; wherein the phase variation of at least a portionof the pixels can be changed over time, wherein the phase variation ischanged in a predetermined fashion wherein the pixels of the spatiallight modulator have an amplitude optical state determined by datasupplied thereto, wherein the predetermined fashion is determined inaccordance with the supplied data, wherein the amplitude optical stateincludes at least two different amplitude optical states, wherein forone of the two different amplitude optical states, the phase variationis of a first amount for a given pixel, and for the next subsequentpixel that is in that same amplitude optical state, the phase variationis of a second amount, with the phase variation continuing to alternatebetween the first and second amounts for subsequent pixels.
 31. A writehead for a holographic data storage system, the write head comprising: aspatial light modulator that is arranged into an array of pixels thatcan each separately modulate light that is directed toward the spatiallight modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through; wherein the phase variation of at least a portionof the pixels can be changed over time, wherein the array of pixels inthe spatial light modulator has a pixel pitch of less than 6 um.
 32. Awrite head for a holographic data storage system, the write headcomprising: a spatial light modulator that is arranged into an array ofpixels that can each separately modulate light that is directed towardthe spatial light modulator and that can vary the phase of light passingtherethrough, the phase variation being dependent on the pixel that thelight passes through; wherein the phase variation of at least a portionof the pixels can be changed over time, wherein the phase variation isdetermined by a phase mask generator that is located in a siliconbackplane with the spatial light modulator. 33-35. (canceled)
 36. Aholographic data storage system, comprising: an optical storage medium;a light source; and a write head including: a spatial light modulatorthat is arranged into an array of pixels that can each separatelymodulate light that is directed toward the spatial light modulator; anda phase mask that is capable of varying the phase of light passingtherethrough and directed toward the spatial light modulator, the phasevariation being dependent on a particular portion of the phase mask thatthe light passes through, wherein the phase variation of at least aportion of the phase mask can be changed over time; wherein the writehead is receptive of light from the light source and the write headselectively directs light to the optical storage medium.
 37. Aholographic data storage system, comprising: an optical storage medium;a light source; and a write head including a spatial light modulatorhaving an array of pixels, each of the pixels in the array beingindependently switchable between a high optical throughput state and alow optical throughput state in response to data supplied thereto, eachof the pixels in the high optical throughput state further beingswitchable between a first optical phase state and a second opticalphase state in a predetermined fashion in accordance with the supplieddata; wherein the write head is receptive of light from the light sourceand the write head selectively directs light to the optical storagemedium.
 38. A method of recording data in an optical storage medium,comprising: providing a source of light; modulating the light on apixel-by-pixel basis across a two-dimensional array of pixels; varyingthe phase of the light on a pixel-by-pixel basis across thetwo-dimensional array of pixels; and interfering the modulated andphase-varied light with a reference light at an optical storage medium.39. A method as defined in claim 38, wherein the light from the sourceof light is divided into a signal beam and a reference beam, the signalbeam then being modulated and phase-varied and the reference beam beingthe reference light.